Currently, active matrix organic light emitting device (“AMOLED”) displays are being introduced. The advantages of such displays include lower power consumption, manufacturing flexibility and faster refresh rate over conventional liquid crystal displays. In contrast to conventional liquid crystal displays, there is no backlighting in an AMOLED display as each pixel consists of different colored organic light emitting devices (e.g., red, green and blue) emitting light independently. The organic light emitting diodes (OLED) emit light based on current supplied through a drive transistor. The drive transistor is typically a thin film transistor (TFT) fabricated from either amorphous silicon or polysilicon. The power consumed in each OLED has a direct relation with the magnitude of the generated light in that OLED.
The drive-in current of the drive transistor determines the pixel's luminance and the surface (aperture) of the actual OLED device determines the pixel's OLED lifetime. AMOLED displays are typically fabricated from the OLED, the drive transistor, any other supporting circuits such as enable or select transistors as well as various other drive and programming lines. Such other components reduce the aperture of the pixel because they do not emit light but are needed for proper operation of the OLED.
Generally color displays have three OLEDs arranged in a “stripe” for each pixel 10 as shown in FIG. 1A. The pixel 10 in FIG. 1A is a bottom emission type OLED where the OLEDs are fabricated on the substrate of the integrated circuit where there is no other components such as transistors and metal lines. The pixel 10 includes OLEDs 12, 14 and 16 and corresponding drive transistors 22, 24 and 26 arranged in parallel creating a “stripe” arrangement. Parallel power lines 32, 34 and 36 are necessary to provide voltage to the OLEDs 12, 14 and 16 and drive transistors 22, 24 and 26. The OLEDs 12, 14 and 16 emit red, green and blue light respectively and different luminance levels for each OLED 12, 14 and 16 may be programmed to produce colors along the spectrum via programming voltages input from a series of parallel data lines 42, 44 and 46. As shown in FIG. 1A, additional area must be reserved for a select line 50 and the data lines 42, 44 and 46 as well as the power lines 32, 34 and 36 for the OLEDs 12, 14 and 16 and the drive transistors 22, 24 and 26. In this known configuration, the aperture of the integrated circuit of the pixel 10 is much less than the overall area of the integrated circuit because of the areas needed for the drive transistor and power and data lines. For example, in producing a shadow mask for fabricating such an integrated circuit for the pixel 10, the distance between two adjacent OLEDs such as the OLEDs 12 and 14 and the OLED size is significant (larger than 20 um). As a result, for high resolution display (e.g. 253 ppi with 33.5 um sub pixel width), the aperture ratio will be very low.
FIG. 1B shows a circuit diagram of the electronic components, namely the OLED 12, the drive transistor 22, the power input for the drive voltage line 32 and the programming voltage input 42 for each of the color OLEDs that make up the pixel 10. The programming voltage input 42 supplies variable voltage to the drive transistor 22 that in turn regulates the current to the OLED 12 to determine the luminance of the OLED 12.
FIG. 1C shows the cross section for the conventional bottom emission structure such as for the pixel 10 in FIG. 1A. As is shown, OLED 12 is fabricated to the side of the other components on the substrate in an open area. Thus, the OLED light emission area is limited by the other components in the pixel. A common electrode layer 70 provides electrical connection to the OLED 12. In this case, the current density is high because of the limited area for light emission. The OLED voltage is also high due to higher current density. As a result, the power consumption is higher and the OLED lifetime is reduced.
Another type of integrated circuit configuration for each of the OLEDs that make up the pixel involves fabricating the OLED over the backplane components (such as transistors and metal traces) and is termed a top emission configuration. The top emission configuration allows greater surface area for the OLED and hence a higher aperture ratio, but requires a thinner common electrode to the OLEDs because such an electrode must be transparent to allow light to be emitted from the OLEDs. The thin electrode results in higher resistance and causes significant voltage drop across this electrode. This may be an issue for larger area displays which in nature need a larger area common electrode.
Therefore, currently, the apertures of pixels for OLED displays are limited due to the necessity of drive transistors and other circuitry. Further, the aperture ratios of the OLEDs in OLED displays are also limited because of the necessity to have a minimal amount of space between OLEDs due to design rule requirements. Therefore, there is a need for increasing the aperture ratios of OLED based integrated circuit pixels for higher resolution displays.